1. Field of the Invention
The invention relates to a polarizer for use in dual polarized antennas fed by parallel plate waveguides. These antennas are often used in applications where an antenna with an elongated aperture is required. Important examples are low profile tracking antennas for satellite communication to/from moving vehicles (automobiles, boats and airplanes).
2. Brief Description of Related Art
It is often necessary in communication systems to feed or receive dual polarized signals to or from the antennas. The two polarizations allow two separate signals to be used at the same frequency and time. It is also necessary to separate the two signals in the circuitry attached to the antenna.
One device which is commonly used to both separate the signals and produce good quality circular polarization is the septum polarizer. In its usual form, this polarizer consists of two rectangular waveguides which are placed “piggy-back”, one on top of the other, so that they share a common broad wall. This wall is cut away to form a shaped taper so that at the end of the taper the cavity enclosed by the other walls defining the waveguides become square in shape. Some designs cut the wall in steps. Others use a smooth taper. The operation and design of this type of device has been discussed in the literature. See “A Wide-Band Square-Waveguide Array Polarizer” by Ming Hui Chen and G. N. Tsandoulas IEEE APS Transactions May 1973 pp 389-391. See also “A New Type of Circularly Polarized Antenna Element” by D. Davis, O. J. Digiandomenico and J. A. Kempic, in G-AP Symp. Dig., 1967 pp. 26-33. 33.
The septum polarizer has three physical ports, i.e., two rectangular waveguides and one square waveguide. However, it has four electrical ports since the square waveguide can support two independent signals with orthogonal polarizations. It is possible to design the taper in the common wall so that the signals in the two rectangular waveguides are well isolated from each other. At the same time, the two polarizations in the square waveguide are also well isolated. Essentially, the signal in one of the rectangular waveguides couples to only one of the polarizations in the square waveguide. Similarly, the signals in the other rectangular waveguide couple to the other polarization in the square waveguide. Usually, the device is designed so that the two orthogonal polarizations in the square waveguide are circularly polarized, or nearly so.
In a number of antenna applications, it is necessary to use an elongated aperture where one dimension of the aperture is much larger that the other. Antennas used in low profile tracking applications, such as those mounted on moving vehicles, are good examples. In these applications, it would be useful to be able to feed the antenna with a parallel plate waveguide. The signals in the waveguide can be collected or injected via an array of probes or by use of a parabolic reflector. An example of this is the invention in U.S. Pat. No. 2,638,546. This type of antenna can be manufactured inexpensively and can be made to have high aperture efficiency. However, this antenna is usually only used with a single linear polarization. The electric field is polarized perpendicular to the metal plates forming the parallel plate waveguide. With the addition of an external polarizer, it can also be used in a single circularly polarized mode.
There are two difficulties in using the parallel plate waveguide in a dual polarized manner. If the spacing between the plates is separated wide enough to allow two orthogonal modes to propagate, a third undesired mode can propagate. This mode is polarized in the same direction as the original mode i.e. perpendicular to the plates but has an anti-symmetric distribution across the guide. Also, the two desired modes behave very differently, they have very different propagation constants and wave impedances.
The design of a feed network that would work well for both desired modes and not produce the undesired mode is a very challenging problem. An alternative is to produce a device, similar to the rectangular waveguide septum polarizer, which has two identical piggy-back waveguides which launch/receive the two dissimilar parallel plate modes in an orthogonal manner. Now the signals in the two identical waveguides can be combined/divided in separate but parallel circuits. The invention disclosed performs this exact function.